If you factory reset a phone and do not know the previous Google account credentials, the phone will lock. Technicians often download a specific version of Google Account Manager (usually version 6.0.1 or similar for older Androids) to access the phone's settings during setup.
Always verify:
To manage your account security settings: google google account manager
This write-up focuses on the latter: the as an Android system component. If you factory reset a phone and do
Because it is a system service, there is no launcher icon. To manage it: google google account manager
For most users, the solution is to of the Account Manager via system settings, then re-add their Google account. For developers, it’s a reminder to handle token refresh failures gracefully in apps using Google Sign-In.